Mold for improved control of heat transfer in casting plate or strip products

ABSTRACT

An open-ended mold is disclosed for continuously casting metal plate or strip, the mold having a graphite casting cavity liner formed of two parallel plates separated at two of their opposing edges with spacer members so as to define an open ended cavity of substantially rectangular cross section, and two cooled metal jackets each tightly clamped against a liner plate. The two principal directions of heat transfer from the mold cavity defined by the liner are through the respective oppositely disposed graphite plates into the adjacent cooled jackets. Improved control over the rate of heat transfer from the mold cavity is obtained by providing a slight convex curvature or crown of suitable height on either the internal surfaces of each cooled jacket or the adjacent external surfaces of the liner plates so that when a crowned surface is tightly clamped against an adjacent flat surface positive control is maintained over the area of contact between the surfaces during casting despite the inherent tendency of the graphite liners to bow to the mold cavity when in contact with hot metal due to thermal expansion.

United States Patent [72] Inventors Robert E. Eppich Southiield; Fred J.Webbere, Orchard Lake, Mich.

[21] Appl. No. 823,706

[22] Filed May 12, 1969 [45] Patented May 25, 1971 [73] Assignee GeneralMotors Corporation Detroit, Mich.

[54] MOLD FOR IMPROVED CONTROL OF HEAT TRANSFER IN CASTING PLATE ORSTRIP PRODUCTS 5 Claims, 6 Drawing Figs.

[52] US. Cl 164/283,

[51] Int. Cl B22d 11/00 [50] Field of Search 164/82,

[56] References Cited UNITED STATES PATENTS 3,410,331 11/1968 Miller eta1. 164/51 3,412,784 11/1968 Wieland 249/134X 3,502,135 3/1970 Wertlil64/82X 3,511,305 5/1970 Wertli 3,519,062 7/1970 Wertli PrimaryExaminerl. Spencer Overholser Assistant ExaminerR. Spencer AnnearAnorneysWilliam S. Pettigrew and George A. Grove ABSTRACT: An open-endedmold is disclosed for continuously casting metal plate or strip, themold having a graphite casting cavity liner formed of two parallelplates separated at two of their opposing edges with spacer members soas to define an open ended cavity of substantially rectangular crosssection, and two cooled metal jackets each tightly clamped against aliner plate. The two principal directions of heat transfer from the moldcavity defined by the liner are through the respective oppositelydisposed graphite plates into the adjacent cooled jackets. Improvedcontrol over the rate of heat transfer from the mold cavity is obtainedby providing a slight convex curvature or crown of suitable height oneither the internal surfaces of each cooled jacket or the adjacentexternal surfaces of the liner plates so that when a crowned surface istightly clamped against an adjacent flat surface positive control ismaintained over the area of contact between the surfaces during castingdespite the inherent tendency of the graphite liners to bow to the moldcavity when in contact with hot metal due to thermal expansion.

MOLD FOR IMPROVED CONTROL OF HEAT TRANSFER IN CASTING PLATE R STRIPPRODUCTS The subject invention is related to the continuous casting offlat plate or strip products through an open-ended mold. Moreparticularly the subject invention is related to a mold structure formedof a liner inset and an adjacent cooled metal jacket wherein improvedmeans for controlling the surface contact between the jacket and linerand therefore the rate of heat transfer therebetween is obtained.

It is now common metallurgical practice to continuously cast strip orplate products ofiron, steel, copper, aluminum or the like through asuitable open ended mold in either a vertical or horizontal mode.Typically the mold contains a liner portion of graphite or like materialand a cooled jacket member in thermal contact with the liner. Graphiteis frequently employed liner material because it has a very high meltingpoint, has a relatively high coefficient of thermal conductivity andprovides lubricity for the withdrawal of the cast product from the mold.The metal jacket is frequently formed of copper base alloys because oftheir high thermal conductivity and is generally cooled by the passageof water or other suitable fluid therethrough. Having had a certaindegree of success in the continuous casting of metal products there isnow interest in providing molds which permit a substantial increase inthe rate of casting and/or an improvement in the metallurgical structureof the cast product. One example of a situation in which both desiderataare sought is in the casting of certain aluminum-babbitt alloys whereinhigh casting rates are desired in combination with the high degree ofcontrol over the character of the dispersion of the babbitt phase in thealuminum matrix. US. Pat. No. 3,410,331 described such alu minum-babbittalloys and a method of casting the same in a horizontal mode to obtain adesired gradient in the concentration of the lead babbitt phasethroughout the aluminum matrix.

It is an object of the present invention to provide an openended moldfor the continuous casting of plate or strip product, the mold havingstructural means for improved control over the rate of heat transferfrom the mold cavity so that the rate of casting of the strip can besubstantially increased and/or improved metallurgical quality of thecast product can be obtained.

It is a more specific object of the present invention'to provide a moldstructure for casting metal product of substantially rectangular crosssection, the mold having a graphite liner portion and a cooled jacketmember wherein the character of the engaging surfaces of the graphiteliner and the cooled jacket are arranged and constructed to provide anincreased and more affirmatively controlled rate of heat transfer fromthe mold cavity through the graphite liner to the cooled jacketnotwithstanding the inherent tendency of the graphite liner to bowinwardly away from the jacket due to thermal expansion.

In accordance with a preferred embodiment of our invention theseandother objects are accomplished by forming a liner of graphite orother suitable material comprising two parallel plates of suitabledimensions separated at two opposite edges by spacer members to definean open-ended mold cavity rectangular in cross section. Two metaljackets adapted for the passage of cooling water or other suitablecooling fluid are provided and adapted to be clamped against the twoliner plates. Either the outer surface of a liner plate or the adjacentsurface of the cooled jacket member is prepared with a convex curvature(or crown) of predetermined curvature and height in a manner that willsubsequently be described in detail. Preferably, the maximum height ofthe convex surface is adjacent the center of the mold cavity. When thecrowned surface is of suitable height and the jacket members and linerare tightly bolted or clamped together, improved control over the rateof heat transfer is realized during casting in the two opposingdirections extending from the mold cavity through the two liner platesinto the cooled jacket members. Surface contact is maintained betweenthe convex surface and the adjacent planar surface, and thereforecontrol over heat transfer is attained at the surface boundaries of thecooled jacket and the graphite liner, despite the inherent tendency ofthe liner plates to bow inwardly due to the relatively high temperatureof the metal inthe mold cavity. A beneficial increase is obtained in theremoval of heat from the mold cavity, particularly at the centerportions of the two large surfaces of the flat product.

These and other objects and advantages of the subject invention will bemore apparent from a detailed description thereof which follows. In thedescription reference will be made to the attached drawings, notnecessarily drawn to scale, in which:

FIG. I is an elevation view, partly in section, of a suitable holdingfurnace and attached horizontally disposed open ended mold arranged andconstructed in accordance with the practice of our invention;

FIG. 2 is a sectional view of a mold of our invention taken along line2-2 of FIG. 1 showing a convex surface provided on the coolingjacketsofthe mold;

FIG. 3 is a sectional view of the mold as in FIG. 2 except a differentembodiment is shown in which the convex surface is provided on the linerplates;

FIG. 4 is a plan view of rectangular cast strip produced by molds inwhich only flat mold surfaces are employed;

FIG. 5 is a plan view of cast strip produced by a mold of our invention;and

FIG. 6 is an elevation view, somewhat schematic in nature, depictingapparatus suitable for providing a crown surface on a mold member inaccordance with our invention.

In FIG. I is shown an induction melting or holding furnace 10 in which areservoir 12 of molten metal is maintained at a suitable castingtemperature. The reservoir 12 is arranged and constructed in theembodiment shown so that molten metal may be withdrawn from an outlet 14at the bottom thereof through a horizontally disposed open-ended mold16. Depending upon the capacity of the reservoir 12, a cast strip 22 ofsubstantial length, up to several hundred feet or more, may be producedfrom a single melt. The mold is detachable joined to the inductionfurnace 10 by suitable bolt means 18, the details of which are notcritical to the practice of the subject invention. Mold 16 comprises aliner 20 and cooling jackets 28. The liner is formed of graphite orother suitable material and defines an open ended mold cavity (shown inFIGS. 1, 2 and 3 to be filled with molten or solidified metal) and whichis substantially rectangular in a cross section transverse to thedirection of flow of the cast metal and withdrawal of the cast stripproduct 22. The peripheral surface of a transverse cross section of theliner 20 is also generally rectangular in configuration. That is atleast the top and bottom surfaces of the liner are planar unless theyare provided with a slight convex curvature or crownas will bedescribed. The character of the outer side surfaces of the liner is notcritical to the practice of this invention. Since it is generallynecessary to periodically replace the liner or insert portion of acontinuous casting mold, for simplicity of construction and ease ofassembly the liner is preferably formed of two substantially rectangularflat plates 24 separated with suitable spacer members 26 at thelongitudinal edges thereof (FIGS. 2 and 3). Two cooling jacket members28 of copper, brass or the like are formed and bolted 36 one each to theliner plates 24. The cooling jackets 28 are provided with internalpassages 30 defined by webs 52 for the flow of cooling water or othersuitable cooling fluid. Preferably, the cooling water or other fluidtraces a curvilinear path through each of the jackets coursing back andforth several times across the width of the jacket as it progresses fromone end of the mold 16 to the other.

In FIGS. 2 and 3 it is seen that each jacket member 28 has an innersurface 32 engaging the adjacent outer surface 34 of the graphite liner.In accordance with our invention the engaging surfaces are characterizedin that either the liner surface 34 or the jacket surface 32 is shapedinto a smooth convex curvature or crown and the other surface issubstantially flat. As shown in FIG. 2 the crown has been formed on theinterior surface 32 of each of the jacket members 28. In general it ispreferred that the crown be formed on the inner surface 32 of the jacketmember 28 because the liner member 20 must be replaced periodically anda new crown surface would have to be prepared in each instance. FIG. 3illustrates the mold 16' of our invention in which the crown surfaceshave been alternatively provided on the outer surface of the linerplates 34'. In

the assembled position, ready for casting operation, the liner plates 24are assembled with the spacers 26 therebetwcen at the longitudinal edgesand mold jackets 28 are placed above and below the graphite liner 20.The complete assembly is either bolted 36 together as shown or clampedtogether by C- clamps or other suitable device.

By way of example the mold of our invention may be employed to caststrip 4 to 8 inches wide and three-eighth inch to 1 inch thick. Theheight of the spacer members 26 generally determines the thickness ofthe cast strip 22 and the width of the spacer member provides thesupport for the edges of the plate. In producing cast strip of about 5to 7 inches in width, we have found that crowns of 0.005 to 0.02 inchmaximum height, the center of the crown being located at a point abovecenter of the cast strip, are generally suitable.

In the startup of a casting operation the outlet opening of the mold isplugged until the molten charge can be prepared and heated to a suitablecasting temperature. The molten metal is permitted to flow into theinlet opening 54 of the horizontally disposed mold 16 and heat isextracted therefrom through the liner 20 into the cooled metal jacket28. The metal solidifies and is then withdrawn through the outlet 56 ofthe mold. In general, the cast strip is not withdrawn in a continuousmotion from the mold but is initially advanced a predetermined incrementover a brief period of time and then stopped for a predetermined periodof time. In some instances it may be desirable to push the cast stripback into the mold a fraction of the distance it has been withdrawnprior to further advancement thereof. These features of the continuouscasting process are generally recognized by those skilled in the art andmay be employed with our improved mold. The utility of our mold is notdependent on a particular casting cycle or cast product removal cycle.

During the operation of the mold the highest temperatures are obtainedwithin the mold cavity. Heat is transferred from the molten metal in themold cavity through the graphite liner, through the inner portion of thecooled jacket and to the circulating cooling fluid. Of course, there isa sustantial temperature differential across each of the mold componentsbetween the mold cavity and the cooling fluid. Because only its cavitydefining surfaces are in contact with the hot metal there is a definitetendency for the liner plates to bow inwardly into the cavity. There isalso generally a lesser tendency (because it is at a lower temperature)for the adjacent surface of the jacket to bow as well. When the linerbows inwardly it tends to move out of physical contact with the adjacentcooling jacket adversely affecting heat transfer therebetween. In priorart continuous casting molds reliance has been placed on the tendency ofboth the jacket and the liner to bow inwardly to a similar extent and onadditional clamping forces to maintain some physical contact for heattransfer between the adjacent surfaces. However, we have found thatmarkedly improved casting results are obtained by providing a convexsurface, as above indicated, on either the outer surface of the liner orthe inner adjacent surface of the jacket and then bolting or clampingthe two members securely together.

Preferably, the maximum height of the convex surface is adjacent theapproximate center of the mold cavity and, thus, of the strip to becast. in the continuous casting of flat strip or plate it is desirableand necessary to remove a substantial portion of the heat through themajor surfaces of the mold liner and particularly through the centerportion of these surfaces. Thus, with the crown surface on either theliner or the jacket and the two members bolted together, substantialcontact can be maintained at the center portion of the major surfacearea of the mold despite the tendency of the liner plates to bow awayfrom the jacket in mold operation.

connection with casting aluminum-lead bearing alloys by the methoddescribed in U.S. Pat. No. 3,410,331. In casting twophase alloys of thistype wherein it is desired to obtain a specific reproducible gradient ofone phase within another there is a particular need for obtainingcontrol over the rate of heat transfer from the mold cavity. We foundthat when conventional molds are employed, wherein the engaging surfacesof both the mold liner and the mold jacket are flat, extended parabolicsolidification fronts 38 are visually observable on the surface of thecast strip 22 such as is depicted in FIG. 4. These solidification frontsare formed at each increment of advance of the cast strip. Extendedparabolic solidification fronts indicate that the center ofthe caststrip solidifies significantly later than the edges of the strip andthat the rate of heat removal across the width of the mold cavity is farfrom being uniform. When an attempt is made to increase the casting rateabove a particular relatively low value, tearing is noted at thesolidification fronts in the center of the strip as indicated at 40 inFIG. 4. On the other hand, when the mold of our invention is employedhaving a crowned surface of 0.015 inch height on the cooling jackets,substantially square solidification fronts 42 as illustrated in FIG. 5,were obtained. The squared-off solidification front indicates that therate of heat removal is generally uniform across the whole width of thecasting area and that the molten metal was solidifying to form the caststrip at the same longitudinal position in the mold uniformly across thewidth of the casting. Moreover, we found that significantly highercasting rates could be employed without tearing the strip. Of even morecritical significance in the case of aluminum-lead alloys, however, thanthe improved casting rate is the fact that the subject mold provideduniformity in the concentration of dispersed lead particles at a givenlevel in the casting throughout its cross section.

A few specific examples will further illustrate the utility andadvantages of the subject mold. It was desired to produce cast strip ofaluminum-lead alloy by the process identified in the above-identifiedpatent. The strip was to be 5% inch wide by three-eighth inch thick.Therefore, the span of the graphite liner plate members 24 between thespacers 26 was 5% inch and the height of the spacers was three-eighth ofan inch. The graphite liner plates were 0.350 inch thick. Thecoolingjacket was formed of hard copper. The mold from molten metalinlet to cast product outlet was 9 inches long. Cooling water from acommon source was admitted to both the top and bottom cooling jackets ata rate to each of 300 gallons per hour. The design of the cooling jacketwas such that the streams of cooling water coursed back and forth ninetimes across the width of a jacket in proceeding from the inlet endadjacent the furnace to the outlet end adjacent the mold outlet. A 0.015inch crown 34' was provided on each of the two graphite liner platemembers 24 so that the mold 16' was of the type depicted in FIG. 3. Theheight of the crown is considered to be the difference in height betweenthe edge of the plate and the middle thereof as indicated in FIG. 6, themaximum height of the crown being in the center of the plate adjacentthe center of the mold cavity.

A charge of molten aluminum based, lead containing alloy was preparedand maintained at about 1,700F. in holding furnace 10. A cast strip wasproduced on a continuous basis by advancing the solidified casting at arate of approximately feet per hour in accordance with the followingcycle. The strip was initially advanced 1 inch in a half second andhalted for 2% seconds, the cycle being repeated on a continuous basis.The cast strip was produced having square solidification fronts 42visible in the surface thereof as shown in FIG. 5. The temperature ofthe cast strip emerging from the mold was 320 F. and a temperature risein the cooling water of 30 F. was noted in both the top and bottomcooling jackets.

The experiment was repeated with the same cast alloy under the sameconditions except that flat graphite plates were employed against flatjacket surfaces and the following results were noted. When an attemptwas made to cast at the above rate, 100 feet per hour, parabolicsolidification fronts 38 of the type depicted in FIG. 4 were obtained.Moreover, center tearing 40 as depicted in FIG. 4 was obtained renderingthe plate unusable. The exit temperature of the plate was 400F. and anincrease in only F. was noted in the cooling water temperature whichindicates that less total heat was removed from the metal. Only when therate of casting was reduced to about 60 feet per hour was tear-freestrip produced. Even then, however, the solidification fronts were stillhighly parabolic. While the substantial sacrifice in casting rateemploying the prior art mold is a distinct disadvantage, of morecritical significance with respect to casting aluminumlead alloy was thefinding that the values of the lead content at a particular depth withinthe casting varied from 3-6 percent over the width of the casting. Thatthis variation would be objectionable in a bearing alloy wherein thefunction of the lead is to provide a substantial portion of thelubricating properties can well be appreciated particularly in view ofthe teachings of the above-identified patent.

Thus, we have found that a convex or crowned surface on either the linermember or the adjacent jacket member produces substantial improvement inthe continuous casting of flat plate or strip in terms of casting rateand in terms of metallurgical quality. ln FIG. 6 is depicted a devicewhich is suitable for preparing the crown surfaces required for the moldcomponents in our invention. A commercial grinding machine 44 isavailable on which the grinding wheels 46 vertical position iscontinuously determined by a mechanical follower 48 which traces a pathover a template 50. Thus a curved surface may be produced on either themold liner plate or the mold jacket by traversing the mechanicalfollower 48 over a simply supported single-point loaded beam template 50which has been deflected to produce a crown of the desired height overthe width of the plate. The beam can also be adjusted so that the pointof maximum deflection of the beam will coincide with the longitudinalaxis of the plate or jacket that is being ground. It will be recognizedthat the desired height of the crown on a given mold member will dependon a number of variables including the temperature of the molten metal,the thickness and width of the composite graphite plate, the thicknessof the jacket in contact with the plate and the elastic and thermalproperties of the two contacting members. As indicated above, in castingaluminum-lead alloys 4 to 8 inches in width, employing graphite platesof about 0.350 inch to 0.500 inch in thickness, a crown of 0.005 inch to0.020 inch is satisfactory. However, the height of the convex curvaturein a particular application of our mold irrespective of the metal beingcast or the mode of casting, whether vertical or horizontal, may bedetermined by experiment. The crown permits a marked increase in thermalconductivity from the center of the mold cavity along the full length ofthe mold which is extremely important respect to rectangular strip forhigh casting rates and product quality.

While our invention has been described in terms of a few specificembodiments it will be appreciated that other forms might readily beadapted by those skilled in the art. Therefore the scope of ourinvention is considered limited only by the following claims.

We claim:

1. A mold for the continuous casting of metal comprising a cooled metaljacket disposed in surface to surface contact heat transfer relationshipwith a graphite mold liner,

said liner defining a cavity having a first open end into which moltenmetal is fed and a second open end from which cast product is withdrawn,said cavity being rectangular in cross section in a plane transverse tothe direction of flow of said metal through said cavity,

the said contacting surfaces of said jacket and said liner beingcharacterized in that one of said surfaces is substantially flat and theadjacent contacting surface is convex with respect to said flat surface.the curvature of said convex surface being predetermined such thatcontrolled surface to surface contact is maintained between said jacketand said liner for purposes of efficient heat transfer during theoperation of said mold.

2. A mold for continuous casting of elongated metal strip or plate ofrectangular cross section comprising a graphite mold insert memberhaving a substantially rectangular outer peripheral surface in a planetransverse to the direction of flow of said cast metal and defining amold cavity of rectangular cross section, said cavity having an inletfor the introduction of molten metal and an outlet for the withdrawal ofcast product therefrom, and a cooled metallic jacket engaging at leastthe two sides of greatest area of said rectangular peripheral outersurface,

the two engaging surfaces of said jacket and a said side of said insertbeing characterized in that one of said surfaces is substantially flatand the engaging surface is convex with respect to said flat surface,the curvature of said convex surface being predetermined such thatcontrolled surface to surface contact between said insert and saidjacket is maintained for efficient heat transfer during a castingoperation.

3. A mold for continuously casting flat metal plate or strip comprisingtwo rectangular plates of graphite separated by spacer members at twoopposite edges of said plates so as to define a mold cavity rectangularin cross section and open at two ends,

and a cooled jacket member tightly clamped in heat transfer relationshipto the outer surface of each of said plates to thereby provide a pathfor the efficient removal of heat from said mold cavity through each ofsaid plates into each of said cooled jackets,

the engaging surfaces of a said plate and a said jacket beingcharacterized in that one said engaging-surface is substantially flatand the other is so convex with respect to said flat surface that in theoperation of said mold in a casting process surface contact to apredetermined degree between the said engaging surfaces is maintainedfor purposes of efficient heat transfer despite the tendency of saidplate member to bow into said mold cavity and out of surface contactwith said jacket due to thermal expansion.

4. A mold for continuously casting flat metal plate or strip of up toabout 8 inches in width and of substantially greater but indefinitelength comprising two substantially flat rectangular plates of graphiteseparated by spacer members at two opposite edges of said plates so asto define a mold cavity rectangular in cross section and open at twoends, the width of said cavity being up to about 8 inches, the height ofsaid cavity being up to about 1 inch,

and a cooled metal jacket tightly clamped in heat transfer relationshipto the outer surface of each of said plates to thereby provide a pathfor efficient removal of heat from said mold cavity through each of saidplates into each of said cooled jackets,

the engaging surfaces of a said plate and a said jacket beingcharacterized in that said surface of said plate is substantially flatand said surface of said jacket is prepared with a slight convexcurvature with respect to said flat surface, the maximum height of saidconvex curvature being about 0.005 inch to 0.020 inch, the location ofsaid maximum height being adjacent the center of said mold cavitywhereby in the operation of said mold in a casting process surfacecontact to a predetermined degree between the said engaging surfaces ismaintained for efficient heat transfer.

5. A mold for continuously casting flat metal plate or strip of up toabout 8 inches in width and of substantially greater but indefinitelength comprising two substantially flat rectangular plates of graphiteseparated by spacer members at two opposite edges of said plates so asto define a mold cavity rectangular in cross section and open at twoends, the width of said cavity being up to about 8 inches, the height ofsaid cavity being up to about 1 inch,

and a cooled metal jacket tightly clamped in heat transfer relationshipto the outer surface of each of said plates to thereby provide a pathfor the efficient removal of heat from said mold cavity through each ofsaid plates into each of said cooled jackets,

imum height being adjacent the center of said mold cavity whereby in theoperation of said mold in a casting process surface contact to apredetermined degree between the said engaging surfaces is maintainedfor efficient heat transfer.

1. A mold for the continuous casting of metal comprising a cooled metaljacket disposed in surface to surface contact heat transfer relationshipwith a graphite mold liner, said liner defining a cavity having a firstopen end into which molten metal is fed and a second open end from whichcast product is withdrawn, said cavity being rectangular in crosssection in a plane transverse to the direction of flow of said metalthrough said cavity, the said contacting surfaces of said jacket andsaid liner being characterized in that one of said surfaces issubstantially flat and the adjacent contacting surface is convex withrespect to said flat surface, the curvature of said convex surface beingpredetermined such that controlled surface to surface contact ismaintained between said jacket and said liner for purposes of efficientheat transfer during the operation of said mold.
 2. A mold forcontinuous casting of elongated metal strip or plate of rectangularcross section comprising a graphite mold insert member having asubstantially rectangular outer peripheral surface in a plane transverseto the direction of flow of said cast metal and defining a mold cavityof rectangular cross section, Said cavity having an inlet for theintroduction of molten metal and an outlet for the withdrawal of castproduct therefrom, and a cooled metallic jacket engaging at least thetwo sides of greatest area of said rectangular peripheral outer surface,the two engaging surfaces of said jacket and a said side of said insertbeing characterized in that one of said surfaces is substantially flatand the engaging surface is convex with respect to said flat surface,the curvature of said convex surface being predetermined such thatcontrolled surface to surface contact between said insert and saidjacket is maintained for efficient heat transfer during a castingoperation.
 3. A mold for continuously casting flat metal plate or stripcomprising two rectangular plates of graphite separated by spacermembers at two opposite edges of said plates so as to define a moldcavity rectangular in cross section and open at two ends, and a cooledjacket member tightly clamped in heat transfer relationship to the outersurface of each of said plates to thereby provide a path for theefficient removal of heat from said mold cavity through each of saidplates into each of said cooled jackets, the engaging surfaces of a saidplate and a said jacket being characterized in that one said engagingsurface is substantially flat and the other is so convex with respect tosaid flat surface that in the operation of said mold in a castingprocess surface contact to a predetermined degree between the saidengaging surfaces is maintained for purposes of efficient heat transferdespite the tendency of said plate member to bow into said mold cavityand out of surface contact with said jacket due to thermal expansion. 4.A mold for continuously casting flat metal plate or strip of up to about8 inches in width and of substantially greater but indefinite lengthcomprising two substantially flat rectangular plates of graphiteseparated by spacer members at two opposite edges of said plates so asto define a mold cavity rectangular in cross section and open at twoends, the width of said cavity being up to about 8 inches, the height ofsaid cavity being up to about 1 inch, and a cooled metal jacket tightlyclamped in heat transfer relationship to the outer surface of each ofsaid plates to thereby provide a path for efficient removal of heat fromsaid mold cavity through each of said plates into each of said cooledjackets, the engaging surfaces of a said plate and a said jacket beingcharacterized in that said surface of said plate is substantially flatand said surface of said jacket is prepared with a slight convexcurvature with respect to said flat surface, the maximum height of saidconvex curvature being about 0.005 inch to 0.020 inch, the location ofsaid maximum height being adjacent the center of said mold cavitywhereby in the operation of said mold in a casting process surfacecontact to a predetermined degree between the said engaging surfaces ismaintained for efficient heat transfer.
 5. A mold for continuouslycasting flat metal plate or strip of up to about 8 inches in width andof substantially greater but indefinite length comprising twosubstantially flat rectangular plates of graphite separated by spacermembers at two opposite edges of said plates so as to define a moldcavity rectangular in cross section and open at two ends, the width ofsaid cavity being up to about 8 inches, the height of said cavity beingup to about 1 inch, and a cooled metal jacket tightly clamped in heattransfer relationship to the outer surface of each of said plates tothereby provide a path for the efficient removal of heat from said moldcavity through each of said plates into each of said cooled jackets, theengaging surfaces of a said plate and a said jacket being characterizedin that the said surface of said jacket is substantially flat and thesaid surface of said plate is prepared with a slight convex curvaturewith respEct to said flat surface, the maximum height of said curvaturebeing about 0.005 inch to 0.020 inch, the location of said maximumheight being adjacent the center of said mold cavity whereby in theoperation of said mold in a casting process surface contact to apredetermined degree between the said engaging surfaces is maintainedfor efficient heat transfer.